Novel Distributed Wavelet Transforms and

Routing Algorithms for Efficient Data Gathering

in Sensor Webs

PI: Antonio Ortega, USC

G. Shen S. Lee S.W. Lee S. Pattem A. Tu B. Krishnamachari

Viterbi School of Engineering, University of Southern California

M. Cheng S. Dolinar A. Kiely M. Klimesh

H. Xie

Jet Propulsion Laboratory, NASA

ESTC 2008 - 06/24/08 2

Efficient Sensor Web Communication Strategies Based on

Jointly Optimized Distributed Wavelet Transform and

Routing

Objectives

•Design algorithms that minimize energy consumption by

compressing correlated measurements as data is routed to the sink

•Enable nodes to reconfigure the network automatically by taking

into account variations in node characteristics

Example of a 2D field measured by a sensor web: (a) true

field, and reconstructed field using (b) distributed

wavelets or (c) quantized data with the same energy

consumption as in (b).

ESTC 2008 - 06/24/08 3

Efficient Sensor Web Communication Strategies Based on

Jointly Optimized Distributed Wavelet Transform and

Routing

Approach

•Develop data compression algorithms that exploit data correlation

-Entropy coding, filter optimization, path merging, joint compression and routing, temporal coding,

compressed sensing

•Implement advances in networking and routing

-Node selection, network initialization, routing optimization, link quality robustness, inclusion of

broadcast nodes, and automatic reconfigurability

•Test these new capabilities

-In the lab, and in a sensor web of about 100 nodes in an outdoor realistic environment for an

extended period of time

Work to Date

•Data compression algorithms

-Entropy coding, path merging, joint compression and routing, temporal coding, compressed sensing

•Advances in networking and routing

-Node selection, routing optimization, inclusion of broadcasts

•In lab experiments

-Preliminary experimental results for small, in lab networks

ESTC 2008 - 06/24/08 4

Presentation Outline

1.Related pre-AIST work

2.Data Compression

•Entropy Coding

•Spatio-temporal transforms and coding

•Spatio-temporal subsampling

•Compressed sensing

•Tree based 2D wavelet transforms

3.Networking

•Joint routing and transform optimization

•Inclusion of broadcast nodes

•Erasure Correcting Codes

4.Mote Implementation

•In lab implementation (tree based 2D wavelet)

ESTC 2008 - 06/24/08 5

Pre-AIST Joint Routing and Compression Technique

(Ciancio, Ortega, Pattem, Krishnamachari – USC)

•Unidirectional 5/3 lifting transform along routing paths [1]

•Heuristic for dealing with merged paths for 2-D networks [2]

•Transform optimization per path

•Pros:

-Unidirectional computation (no backward transmissions)

-Path-wise transform optimization

-Practical alternative to existing methods

•Cons:

-Overhead from heuristic merging technique (not critically sampled)

-Only exploits path-wise correlation

-Optimization only path-wise, does not extend to 2D transforms

[1]. A Ciancio and A. Ortega, “A distributed wavelet compression algorithm for wireless multihop

sensor networks using lifting”, ICASSP’04.

[2]. A. Ciancio, S. Pattem, A. Ortega, B. Krishnamachari, “Energy-efficient data representation

and routing for wireless sensor networks based on a distributed wavelet compression

algorithm”, IPSN’06.

ESTC 2008 - 06/24/08 6

Goal: Use entropy coding (data compression) to minimize cost of

transmitting values needed to compute a Discrete Wavelet Transform

(DWT) in the sensor web. This reduces the energy required to achieve

a given level of quality in the reconstructed data.

Motivation: Combined distributed DWT and entropy coding enables joint

compression of the data generated by different nodes as the

information accumulates over the routing path.

Results:

•Devised a general purpose entropy coding method for our transforms

•Details found in our NSTC 07 Paper [1][1]. G. Shen, et al, “A distributed wavelet approach for efficient information

representation and data gathering in sensor webs ”, NSTC’07.

Entropy Coding

(A. Kiely, M. Klimesh, H. Xie - JPL)

ESTC 2008 - 06/24/08 7

Goal:

•Combine temporal and spatial coding to minimize overall data transmission for

further energy reduction

•Consider both temporal and spatial correlation for node selectionMotivation:

•Most existing work focuses on spatial correlations only

•Data collected at each node exhibits high temporal correlation

•Temporal processing is local and far cheaper than spatial compression, and so

should be fully exploited to minimize the transmission cost

Two notable exceptions:

•Lightweight Temporal Coding [1]

•Distributed Predictive Coding [2][1]. T. Schoellhammer, B. Greenstein, E. Osterweil, M. Wimbrow, and D. Estrin, “Lightweight

temporal compression of microclimate datasets”, LCN'04.

[2]. A. Saxena and K. Rose, “Distributed predictive coding for spatio-temporally correlated

sources”, ISIT 2007.

Spatio-Temporal Coding (H. Xie – JPL)

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Key Observation:

•In data aggregation-based compression, data is transmitted through multiple hops along a

predefined routing path, and compressed jointly it flows towards the sink

•To encode data at a node for a given time instance, we can use all the information from the current

node / time instance along with data from previous nodes / time instances (see figure below)

Information flow along a 1D path in an aggregation-based data transmission system.

Spatio-Temporal Coding

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Assumptions

•Spatial routing path is established

•Latency introduced by local temporal

processing is tolerable

Approaches

•Separable wavelet transform

•Perform a single stage 3/5 reversible

integer DWT on data sequence at each

node to exploit temporal redundancy

•Perform spatial compression using

distributed wavelet transform and entropy

coding

•Adaptive filtering (work in progress)

•Each sample value is predicted from the

historical data

•The difference between the estimate and

the actual value is encoded and

transmitted

•The estimation error is also used to update

the filter weights

Rate-Distortion Performance

MSE Distortion

Average Transmission Cost

(Bits/Coefficient)

(a)

(b)

(c)

This rate-distortion graph shows the benefit of (a) Spatio-temporal

coding using 2D wavelet transform and (b) Spatial compression only,

compared to the baseline approach of (c) entropy coding quantized

sample differences (spatial only).

Example: 10-bit source data as quantized version of 2D

second-order Auto Regressive process with poles at

0.99e±j/64

Spatio-Temporal Coding

ESTC 2008 - 06/24/08 10

•Spatio-temporal sampling patterns may lead to lower transmission cost

for same quality

•Benefits depend on spectral characteristics of data

Spatio-Temporal Sampling

(S. Lee and A. Ortega – USC)

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Spatio-Temporal Sampling

Results

–

Real World Data : VTB data

[1]

There is max 2.6dB gain vs. temporal-only case in cost-PSNR sense.

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[1] Jeongyeup Paek, Omprakash Gnawali Ki-Young Jang, Daniel Nishimura, Ramesh Govindan, John Caffrey, Mazen Wahbeh, Sami Masri, A

Programmable Wireless Sensing System for Structural Monitoring, In: 4th World Conference on Structural Control and

Monitoring(4WCSCM), San Diego, CA, July 2006

ESTC 2008 - 06/24/08 12

Transforms and Routing

•Problem definition:

-What data should be gathered from each node, how should it be

aggregated and transferred to the fusion node, how should it be

reconstructed

•Two major approaches:

-Traditional techniques use data from all nodes, and reconstruct snapshots

of the state of field

a.Can handle any type of data

b.Exact reconstruction up to quantization error

-We are exploring new techniques (i.e., compressed sensing, spatio-

temporal sampling), where:

a.Some form of undersampling is used

b.Exact reconstruction only for classes of signals (sparse, band-limited)

ESTC 2008 - 06/24/08 13

Compressed Sensing for Sensor Networks

(S. Lee, S. Pattem, B. Krishnamachari, A. Ortega)

Overview:

•Compressed Sensing (CS) is a technique capable of representing an N-

length signal (which is K-sparse) using only M << N measurements

Goal:

•Design measurement matrices that lead to efficient routing while also

maintaining a high level of reconstruction quality

Experimental Observations:

•Our previous method [1] designed routing/measurement matrices that are

highly “incoherent” with the assumed signal basis (Fourier, Haar, etc)

-However, correlation between coherence and reconstruction quality is low

•Spatial downsampling (DS) with CS is more efficient both in terms of

reconstruction quality and energy cost

[1]. G. Shen, et al, “A distributed wavelet approach for efficient information representation and data

gathering in sensor webs ”, NSTC’07.

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Compressed Sensing for Sensor Networks

•DS consumes less energy for the same level of reconstruction quality

than DRP and SRP

-AR data and DCT / Multi-level Haar basis

-DS projection is highly incoherent with DCT basis and Haar basis.

Energy ratio vs. SNR of DS and SRP projections for AR data.

DRP is out of range due to very high energy cost.

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Compressed Sensing for Sensor Networks

•CS with DS projection can provide a higher SNR at the same cost for

the low SNR region than 2D wavelet transform.

-AR data and DCT basis for CS

-CS has limited achievable SNR (unless the number of projections increases

significantly.)

CS with DCT basis vs. 2D wavelet transform.

ESTC 2008 - 06/24/08 16

Compressed Sensing for Sensor Networks

•Conclusions

-Coherence is not accurate enough indicator for the reconstruction to

consider as design metric for measurement matrix

-Downsampling with CS is efficient in terms of reconstruction quality and

energy consumption

•Future work

-Extending to general topologies (currently using square grid)

ESTC 2008 - 06/24/08 17

Tree Based 2D Wavelet Transforms

(G. Shen, A. Ortega - USC)

Goal: Design and optimize a 2D transform

•Invertible, critically sampled, and unidirectional

•Exploits 2D correlation (not just path-wise, 1D correlation)

•Lifting filters arbitrary to permit filter optimizationMotivation:

•More de-correlation w/ 2D transform than path-wise transform [1]

•Existing 2D transforms require backwards transmissions [2]Our Proposed Method:

•Lifting transform along an arbitrary tree [3]

•Transform optimization via dynamic programming along a tree[1]. A Ciancio and A. Ortega, “A distributed wavelet compression algorithm for wireless multihop sensor

networks using lifting”, ICASSP’04.

[2]. R. Wagner, H. Choi, R. Baraniuk, V. Delouille, “Distributed wavelet transform for irregular sensor network

grids”, IEEE SSP’05.

[3]. G. Shen and A. Ortega, “Optimized distributed 2D transforms for irregularly sampled sensor network grids

using wavelet lifting”, ICASSP’08.

ESTC 2008 - 06/24/08 18

Tree Based 2D Wavelet Transforms

Lifting Transform Design (Split Design)

•Split sensors into even and odd nodes according to depth in tree

•Sequence of splitting trees across multi-levels (derive T

j from Tj-1

)

ESTC 2008 - 06/24/08 19

Tree Based 2D Wavelet Transforms

Lifting Transform Design (Filter Design)

•Can be computed in a variety of ways (planar regression, etc)

•We use simple averaging and smoothing ideas

Lifting Transform Computation

•Explicitly separate terms for parents

and children

•Permits unidirectional transform computation (no backwards tx)

Children of m in

splitting tree Tj

Parent of m in

splitting tree T

j

ESTC 2008 - 06/24/08 20

Optimized Unidirectional 2D Transforms on

Arbitrary Trees

Unidirectional Transform Computation

ESTC 2008 - 06/24/08 21

Tree Based 2D Wavelet Transforms

Transform Optimization (Cost Minimization for Fixed Distortion)

•Formulate as a Forward Dynamic Program

•Define the following quantities:

-S = {1, 2, …, J} the set of coding schemes (levels of decomposition)

- the cost to transition from level i at n to level j at parent of n

- the optimal cost to arrive at level j at node n from its children

•We then have:

•The optimal solution is found using the results of Algorithm 1

ESTC 2008 - 06/24/08 22

Tree Based 2D Wavelet Transforms

Results (Highly Correlated AR-2 Data, 100 Nodes)

ESTC 2008 - 06/24/08 23

Joint 2D Transform and Routing Optimization

(G. Shen, A. Ortega - USC)

Goal:

•Find routing trees that jointly optimize routing and compressionKey Observations:

•Shortest path routing trees (SPT) provide minimum total distance, not

always minimum per hop distance (for higher correlation)

•Hurts coding efficiency if data along tree is not well correlated

•Per hop correlation is higher along minimum spanning tree (MST)

•Tradeoff between low total multi-hop dist. and high per hop corr.

ESTC 2008 - 06/24/08 24

Joint 2D Transform and Routing Optimization

Optimization Problem:

•Find a spanning tree that minimizes total cost for a given distortionOur Proposed Method:

•Find an optimized combination of trees, i.e., MST and SPT [1]

•Useful approximation to true optimal tree

-Number of spanning trees is very large (Matrix-Tree Theorem)

-Can exploit tradeoff by combining min. cost routing (SPT) with high data

correlation tree (correlation- based MST)

Two Main Methods:

•Search over all combinations of SPT and MST

•Start from an SPT, search from greatest depth nodes down to the sink

-Exchange parent to that of parent in MST

-If new tree is valid and cost is lower (for same distortion), change the parent from the

SPT to that in the MST

[1]. G. Shen and A. Ortega, “Joint routing and 2D transform optimization for irregular sensor

network grids using wavelet lifting”, IPSN’08.

ESTC 2008 - 06/24/08 25

Joint 2D Transform and Routing Optimization

Results for Uniform Network

Edges in MST are

more efficient

Heu. And Opt. differ

by only one edge

ESTC 2008 - 06/24/08 26

Joint 2D Transform and Routing Optimization

Results for Uniform Network (Continued)

Less than 1 dB diff.

between optimal and

heuristic trees

ESTC 2008 - 06/24/08 27

Broadcast Nodes

Goal:

•Design a communication

strategy to exploit these

extra communications

Motivation:

•Broadcast come for free

•The cost of the acquiring

messages at additional

nodes is negligible in many

realistic scenarios

Tree-structure

communication paths

to the sink node

Same network with

possible additional “free”

communications arising

from the broadcast

nature of the links

Broadcast Nodes - Overview

Node communications are not directional; multiple nodes may be able to receive a

single transmission

ESTC 2008 - 06/24/08 28

Exploiting Broadcast Capability

(G. Shen, S. Pattem, A. Ortega – USC)

Goal:

•Utilize broadcast to enhance performance of our current systemMotivation:

•Broadcast allows nodes to transmit data to multiple neighbors

(more than their neighbors defined in a routing tree)

•Can use this to further de-correlate dataOur Proposed Method:

•Start with an initial routing tree T

•Exploit broadcast whenever possible along the tree

-Augment the initial tree T to include broadcasts

-Can even use more general graphs, forward data along T

ESTC 2008 - 06/24/08 29

Exploiting Broadcast Capability

Preliminary Broadcast Technique

•While forwarding along T, even node broadcasts at depth d can reach multiple

odd nodes at depth d+1, d+3, … , d-1, d-3, …

•Exploit these broadcasts by augmenting T into the graph T

A (see figure)

•Compute predicts along TA

and updates along T

Added links

ESTC 2008 - 06/24/08 30

Exploiting Broadcast Capability

Comparisons of Predict Computations

•Having more neighbors can produce lower predict energy

•Lower energy predicts require fewer bits to encode

Predict equations along TPredict equations along T

A

ESTC 2008 - 06/24/08 31

Exploiting Broadcast Capability

Preliminary Results

•Uniform 200 node network with highly correlated AR-2 data

Green – broadcast links

Blue – tree links

ESTC 2008 - 06/24/08 32

Exploiting Broadcast Capability

Preliminary Results (Continued)

ESTC 2008 - 06/24/08 33

Erasure Correcting Codes - Link Quality

Considerations (M. Cheng, S. Dolinar - JPL)

Goal: Study and implement various erasure-correcting codes for sensor

networks that achieve more reliable sensor-to-sensor communications

-Reduces number of retransmissions, resulting in energy savings

-Allows for more robust system performance under varying degrees of link

quality

Motivation: Data transmissions between nodes are subject to erasures

with probability p, resulting in costly retransmissions that increase

overall energy consumption

Current Results:

•Developed software to generate and to simulate performance of:

-State-of-the-art LT rateless codes

-Blake’s new windowed erasure-correcting codes

•Selected LT codes for implementation based on lower complexity and

greater maturity (but not yet implemented)

ESTC 2008 - 06/24/08 34

Mote Implementation

(S. Pattem, B. Krishnamachari – USC)

Implementation Related Goals:

•Provide in-lab testbed for algorithm performance evaluation

•Identify and establish a real environment for real time algorithm

implementation

Current Results:

•Hardware implementation of invertible 2D wavelet

-Tmote Sky devices (CC2420 radio)

•Storage and packetization

•Distributed and flexible operation (any given tree)

ESTC 2008 - 06/24/08 35

Mote Implementation

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ESTC 2008 - 06/24/08 36

Mote Implementation

Results Show:

•Correctness of 2D wavelet implementation

•Tradeoff between cost and reconstruction quality

Looking Ahead

•For wide usage, need an architectural view that defines:

-Where compression fits into standard network stack

-Interactions between compression and networking components

-Modules in compression component

-Robustness mechanisms for distributed compression

ESTC 2008 - 06/24/08 37

Mote Implementation

Our Proposed Architecture

data,

robustness info

data, allow

processing

processed

data

order, role based

processing

structure

metric/goals

consistency

FORWARDING

ROUTING

LINK/NEIGHBOR

TABLE

COMPRESSION

CONFIGURATION

CLUSTERING

COMPUTATION

STORAGE

BIT REDUCTION &

PACKETIZATION

APPLICATION (SENSING)

MAC

measurements

order, role based

requirements

consistency

ESTC 2008 - 06/24/08 38

Future Work

•Data Compression Methods

-Extensions of tree based 2D wavelets (i.e., “Tree-lets”)

-Filter optimization methods

-Extend temporal coding methods to tree based 2D wavelets

•Networking and Routing Methods

-Node selection

-Network initialization

-Link quality robustness

-Automatic reconfigurability

•Real Mote Implementation

-Integrate entropy coding and erasure correcting codes

-Develop software that handles an arbitrary sensor web topology

-Test our algorithms in an outdoor realistic environment

ESTC 2008 - 06/24/08 39

Questions

???

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